Chapter 13 Multiple-Use-Mold Casting Processes EIN 3390 Manufacturing Processes Spring, 2012 - PowerPoint PPT Presentation

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Chapter 13 Multiple-Use-Mold Casting Processes EIN 3390 Manufacturing Processes Spring, 2012

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Title: Chapter 13 Multiple-Use-Mold Casting Processes EIN 3390 Manufacturing Processes Spring, 2012


1
Chapter 13Multiple-Use-Mold Casting
ProcessesEIN 3390 Manufacturing
ProcessesSpring, 2012
2
13.1 Introduction
  • In expendable mold casting, a separate mold is
    produced for each casting and has the following
    disadvantages
  • Low production rate
  • Quality control issues, such as dimensional and
    property variation due to
  • Mold to be crated each time
  • Variation in mold consistency
  • Mold strength
  • Mold moisture content
  • Pattern removal

3
13.1 Introduction
  • Multiple-use molds
  • Assets
  • Higher productivity
  • Good product quality
  • Liability
  • Metal molds are limited to low melting
    temperature nonferrous metals and alloys
  • Part size limitation
  • Higher cost of dies or molds

4
13.2 Permanent-Mold Casting
  • Also known as gravity die casting
  • Mold materials
  • Gray cast iron, alloy cast iron, steel, bronze,
    or graphite
  • Most molds are made in segments with hinges to
    allow rapid and accurate closing
  • Molds are preheated to improve properties
  • Liquid metal flows through the mold cavity by
    gravity flow

5
Permanent Mold Casting
  • Process can be repeated immediately because the
    mold is still warm from the previous casting
  • Most frequently cast metals
  • Aluminum, magnesium, zinc, lead, copper, and
    their alloys
  • If steel or iron is to be used, a graphite mold
    must be used

6
Advantages of Permanent-Mold Casting
  • Near- net shapes
  • Little finish machining
  • Reusable molds
  • Good surface finish
  • Consistent dimensions
  • Directional solidification
  • Fast cooling rate to produce a strong structure
  • Core can be used to increase complexity

7
Disadvantages of Permanent Mold Casting
  • Limited to lower melting temperature alloys
  • High mold costs
  • Mold life is strongly tied to cost
  • Mold life is dependent on the following factors
  • Alloys being cast, especially melting temperature
  • Mold material
  • Pouring temperature
  • Mold temperature
  • Mold configuration
  • High production runs can validate high mold costs
  • Limited mold complexity

8
Low Pressure Permanent-Mold Casting
  • Molds are not permeable
  • Venting in Permanent Casting
  • Slight crack between mold halves
  • Very small vent holes to permit escape of trapped
    air, but not passage of molten metal
  • Design feature affects mold life
  • Difference in section size through mold
  • Removal castings immediately after solidification

9
Permanent Mold Casting
10
Low Pressure Permanent-Mold Casting
  • Low pressure permanent-mold (LPPM) casting
  • Mold is upside down and connected to a crucible
    that contains the molten metal
  • Pressure difference induces upward flow
  • Metals are exceptionally clean because it is fed
    directly into the mold
  • Little or no turbulence during flow
  • No risers, yields gt 85
  • Mechanical properties are about 5 better than
    those of conventional permanent-mold casting.
  • Typical metals cast using low pressure process
  • Aluminum, magnesium, and copper

11
Low-Pressure Permanent-Mold Casting
Figure 13-2 Schematic of the low-pressure
permanent-mold process. (Courtesy of Amsted
Industries, Chicago, IL.)
12
Vacuum Permanent-Mold Casting
  • Atmospheric pressure in the chamber forces the
    metal upward after the vacuum is drawn
  • Thin-walled castings can be made
  • Excellent surface quality
  • Cleaner metals than low pressure
  • Lower dissolved gas content
  • Better mechanical properties than low pressure
    casting
  • Final castings range 0.2 to 5 kg and have better
    mechanical properties than LPPM.

13
Vacuum Permanent-Mold Casting
Figure 13-3 Schematic illustration of vacuum
permanent-mold casting. Note the similarities to
the low-pressure process.
14
13.3 Die Casting
  • Molten metal is forced into the mold under high
    pressure
  • Held under high pressure during solidification
  • Castings can have fine sections and complex
    details
  • Long mold life
  • Typical metals cast
  • Zinc, copper, magnesium, aluminum, and their
    alloys

15
Advantages of Die Casting
  • High production rates
  • Good strength
  • Intricate shapes
  • Dimensional precision
  • Excellent surface qualities
  • Small-medium sized castings

16
Disadvantages of Die Casting
  • High cost for dies
  • Less flexibility for products
  • Limited to small- to medium-sized parts
  • Most for nonferrous metals and alloys

17
Die Modifications and Die Life
  • Die complexity can be improved through the use of
  • Water cooled passages
  • Retractable cores
  • Moving pins to eject castings
  • Die life
  • Limited by erosion and usage temperature
  • Surface cracking
  • Heat checking
  • Thermal fatigue

18
Die Modifications and Die Life
  • Die Materials
  • Harden tool steels, since cast iron cannot
    withstand casting pressures
  • Vary Pressure on Molten Metal during casting
  • Reduce turbulence and air entrapment by lower
    injection pressures, and followed by higher
    pressures after mold has been filled completely
    and metal starts to solidify.

19
Die-Casting Dies
Figure 13-4 Various types of die-casting dies.
(Courtesy of American Die Casting Institute,
Inc., Des Plaines, IL.)
20
Basic Types of Die-Casting
  • Hot chamber castings
  • Gooseneck chamber for molten metal
  • Plunger to control molten metal flow
  • Fast cycling times
  • No handling or transfer of molten metal
  • Cant used for higher-melting-point metals
  • Aluminum tends to pick up some iron of casting
    equipments
  • Used with zinc, tin, and lead-based alloys

21
Die Casting (Hot-Chamber)
Figure 13-5 (Below) Principal components of a
hot-chamber die-casting machine. (Adapted from
Metals Handbook, 9th ed., Vol 15, p. 287, ASM
International, Metals Park, OH.)
22
Basic Types of Die Casting
  • Cold-chamber machines
  • Used for materials not suitable for hot chamber
    machines
  • Separated furnace
  • Drive measured quantity of molten metal into
    unheated chamber by hydraulic plunger
  • Maintain or increase pressure until
    solidification done
  • Typical materials
  • Aluminum, magnesium, copper, and high-aluminum
    zinc
  • Longer operating cycle than hot-chamber
  • High productivity

23
Die Casting (Cold-Chamber)
Figure 13-6 (Above) Principal components of a
cold-chamber die-casting machine. (Adapted from
Metals Handbook, 9th ed., Vol 15, p. 287, ASM
International, Metals Park, OH.)
24
Summary of Die Casting
  • Dies fill so fast with metal, only little time
    for the air in the runner and die to escape
  • Molds offer no permeability
  • Air can become trapped and cause defects
  • Risers are not used because of the high pressures
    used
  • Sand cores cant be used due to high pressures
  • Cast-in inserts can be used
  • High production rates
  • Little post casting finishing necessary

25
Die Casting
26
Die Cast Materials
27
13.5 Centrifugal Casting
  • Inertial forces due to spinning distribute the
    molten metal into the mold cavity
  • True centrifugal casting
  • Dry-sand, graphite or metal mold can be rotated
    horizontally or vertically
  • Exterior profile of final product is normally
    round
  • Gun barrels, pipes, tubes
  • Interior of the casting is round or cylindrical
  • If the mold is rotated vertically, the inner
    surfaces will be parabolic

28
Centrifugal Casting (Horizontal)
  • Specialized equipment
  • Expensive for large castings
  • Long service life
  • No sprues, gates, or risers

Figure 13-8 (Left) Schematic representation of a
horizontal centrifugal casting machine. (Courtesy
of American Cast Iron Pipe Company, Birmingham,
AL.)
29
Centrifugal Casting (Vertical)
Figure 13-9 (Above) Vertical centrifugal casting,
showing the effect of rotational speed on the
shape of the inner surface. Parabaloid A results
from fast spinning whereas slower spinning will
produce parabaloid B.
30
Centrifugal Casting
Figure 13-10 Electrical products (collector
rings, slip rings, and rotor end rings) that have
been centrifugally cast from aluminum and copper.
(Courtesy of The Electric Materials Company,
North East, PA.)
31
Centrifugal Casting
32
Centrifugal Casting
  • Semicentrifugal casting
  • Several molds may be stacked on top of one
    another
  • Share a common basin and sprue
  • Used for gear blanks, pulley sheaves, wheels,
    impellers, etc.

33
Centrifuging Casting
Figure 13-11 Schematic of a semicentrifugal
casting process.
34
Centrifugal Casting
  • Centrifuging
  • Uses centrifugal acceleration to force metal into
    mold cavities that are offset from the axis of
    rotation

35
Centrifuging
Figure 13-12 (Above) Schematic of a centrifuging
process. Metal is poured into the central pouring
sprue and spun into the various mold cavities.
(Courtesy of American Cast Iron Pipe Company,
Birmingham, AL.)
36
13.6 Continuous Casting
  • Used for the solidification of basic shapes for
    feedstock for deformation process such as rolling
    and forging.
  • Can be used to produce long lengths of complex
    cross sections

Figure 13-13 Gear produced by continuous casting.
(Left) As-cast material (right) after machining.
(Courtesy of ASARCO, Tucson, AZ.)
37
13.7 Melting
  • Selection of melting method is based on several
    factors
  • Temperature needed to melt and superheat the
    metal
  • Alloy being melted
  • Desired melting rate and quantity
  • Desired quality of metal
  • Availability and cost of fuels
  • Variety of metals or alloys to be melted
  • Batch or continuous
  • Required level of emission control
  • Capital and operating costs

38
Cupolas
  • Cupola- refractory-lined, vertical steel shell
  • Alternating layers of carbon (cock), iron,
    limestone, and alloy additions
  • Melted under forced air
  • Simple and economical
  • Melting rate can be increased by using hot-blast
    cupolas, oxygen-enriched blasts, or plasma torches

39
Types of Furnaces
  • Indirect Fuel-Fired Furnace
  • Crucibles or holding pots are heated externally
    which in turn heats the metal
  • Low capital and operating costs
  • Direct Fuel-Fired Furnace
  • Similar to small open-hearth furnaces
  • Flame passes directly over metal

Figure 13-14 Cross section of a direct fuel-fired
furnace. Hot combustion gases pass across the
surface of a molten metal pool.
40
Arc Furnaces
Figure 13-15 Schematic diagram of a three-phase
electric-arc furnace.
  • Preferred method for most factories
  • Rapid melting rates
  • Ability to hold molten metal for any period of
    time
  • Greater ease of incorporating pollution control
    equipment

41
Induction Furnaces
  • Rapid melting rates
  • Two basic types of induction furnaces
  • High-frequency (coreless)
  • Contains a crucible surrounded by a water-cooled
    coil of copper tubing
  • High-frequency electrical current induces an
    alternating magnetic field
  • The magnetic field, in turn, induces a current in
    metal being melted
  • Low-frequency (channel-type)
  • Small channel is surrounded by the primary coil
    and a secondary coil is formed by a loop or
    channel of molten metal

42
Induction Furnaces (Coreless)
Figure 13-17 Schematic showing the basic
principle of a coreless induction furnace.
43
Induction Furnaces (Low Frequency)
Figure 13-18 Cross section showing the principle
of the low-frequency or channel-type induction
furnace.
44
13.8 Pouring Practice
  • Ladles are used to transfer the metal from the
    melting furnace to the mold
  • Concerns during pouring
  • Maintain proper metal temperature
  • Ensure that only high-quality metal is
    transferred
  • Pouring may be automated in high-volume,
    mass-production foundries

45
Automatic Pouring
Figure 13-19 Automatic pouring of molds on a
conveyor line. (Courtesy of Roberts Sinto
Corporation, Lansing, MI.)
46
13.9 Cleaning, Finishing, and Heat Treating of
Castings
  • Post-casting operations
  • Removing cores
  • Removing gates and risers
  • Removing fins, flash, and rough surface spots
  • Cleaning the surface
  • Repairing any defects
  • Cleaning and finishing may be expensive, so
    processes should be selected that minimize
    necessary operations

47
Cleaning and Finishing
  • Sand cores may be removed by mechanical shaking
    or chemically dissolved
  • Flash may be removed by being tumbled in barrels
    containing abrasive materials
  • Manual finishing
  • Pneumatic chisels, grinders, blast hoses
  • Porosity at surfaces may be filled with resins
    (impregnation)
  • Pores may also be filled with lower-melting point
    metals (infiltration)

48
Heat Treatment and Inspection of Casting
  • Heat treatments alter properties while
    maintaining shape
  • Full anneals reduce hardness and brittleness of
    rapidly cooled castings
  • Reduce internal stresses
  • Nonferrous castings may be heat treated to
    provide chemical homogenization or stress relief
  • Prepares materials for further finishing
    operations

49
13.10 Automation in Foundries
  • Most manufacturing operations may be performed by
    robots
  • Dry mold, coat cores, vent molds, clean or
    lubricate dies
  • Plasma torches
  • Grinding and blasting
  • Investment casting
  • Lost foam process
  • Casting can be dangerous for workers by
    automating these processes, safety is increased

50
13.11 Process Selection
  • Each casting process has advantages and
    disadvantages
  • Typical requirements
  • Size, complexity, dimensional precision, surface
    finish, quantity, rate of production
  • Costs for materials (dies, equipment, and metal)

Figure 13-20 Typical unit cost of castings
comparing sand casting and die casting. Note how
the large cost of a die-casting die diminishes as
it is spread over a larger quantity of parts.
51
(No Transcript)
52
Summary
  • Variety of casting processes
  • Each has its own set of characteristics and
    benefits
  • Care should be taken in properly selecting a
    casting process to minimize cost while maximizing
    qualities of the finished product
  • Most casting processes may be automated, but the
    process selected determines the quality of the
    finished product
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